How can I create an SSH user who only has permission to access specific folders

This is simple. Just create a new user with it's home directory set to the one you need him to have access to (this command must be run under sudo or in root shell):

adduser --home /restricted/directory restricted_user

This will create an user restricted_user, the directory /restricted/directory and then permissions on the directory will be set so the user can write to it. It won't have an ability to write to any other directory by default.

If you have the directory already, you can run adduser command with a --no-create-home option appended and set permissions manually (also with root privileges), like:

chown restricted_user:restricted_user /restricted/directory
chmod 755 /restricted/directory

If you need to make even world-writable directories unaccessible for this user, there are two variants.

1) If you want to provide an interactive shell session to the user, then consider following this manual on creating a chroot jail (in your /restricted/directory).

After that add following to your sshd_config:

Match user restricted_user
  ChrootDirectory /restricted/directory

2) If you only need him to copy files between his endpoint of connection and your host, everything is much easier. Add these lines in end of your sshd_config:

Match user restricted_user
  ForceCommand internal-sftp
  ChrootDirectory /restricted/directory

Subsystem       sftp    internal-sftp

Then comment out the Subsystem sftp /usr/lib/openssh/sftp-server by placing a hash (#) sign in the start.

After restarting your SSH server (it does not kill interactive sessions on restart, so it is safe even if you misconfigured something; also, does not close your running session before you have checked that you are still able to log in), everything should work as intended.